1. Field of the Invention
The present invention relates to a brake force control apparatus for a vehicle, and more particularly to a brake force control apparatus having a hydraulic braking unit and a regenerative braking unit.
2. Description of the Related Art
Conventionally, as disclosed in Japanese Laid-Open Patent Application No. 7-205800, a brake force control apparatus is known having a hydraulic braking unit and a regenerative braking unit. The hydraulic braking unit generates a hydraulic brake force corresponding to a friction force between a brake pad and a disk rotor by pressing the brake pad against the brake rotor in accordance with a wheel cylinder pressure. On the other hand, the regenerative braking unit generates a regenerative brake force corresponding to a regenerative energy generated in association with a rotation of a driving wheel driven by a motor. The hydraulic brake force and the regenerative brake force are controlled so that the sum thereof is equal to a brake force required by a driver (hereinafter referred to as a required brake force).
When the regenerative brake force is generated, a regenerative energy corresponding to the regenerative brake force is supplied to a battery as a charging current. Therefore, from a viewpoint of maintaining a charging state of the battery, the regenerative brake force is desired to be greater. On the other hand, the maximum regenerative brake force which can be generated by the regenerative braking unit is limited by the maximum regenerative energy which can be received by the battery and by a rotational speed of the driving wheel.
Thus, in the above-mentioned brake force control apparatus, when the maximum regenerative brake force is greater than or equal to the required brake force, the above-mentioned brake force control apparatus generates a regenerative brake force which is equal to the required brake force and sets the hydraulic brake force to be zero. On the other hand, when the maximum regenerative brake force is smaller than the required brake force, the apparatus generates a regenerative brake force which is equal to the maximum regenerative brake force and a hydraulic brake force which corresponds to the difference between the maximum regenerative brake force and the required brake force. In this way, it is possible to generate as large a regenerative brake force as possible.
As mentioned above, the hydraulic brake force is generated by a friction between the brake pad and the brake rotor. Thus, in a state where the hydraulic brake force is not generated, the brake pad and the brake rotor are not in contact with each other. On the other hand, in a state where the hydraulic brake force is generated, the brake pad and the brake rotor are in contact with each other. Accordingly, in the above-mentioned brake force control apparatus, when a relationship of magnitude between the maximum regenerative brake force and the required brake force changes during a brake operation, the hydraulic brake force is intermittently generated due to a transition between a state where the brake pad and the brake rotor are not in contact with each other and a state where the brake pad and the brake rotor are in contact with each other. In this case, when a transition occurs from the former state to the latter state, the hydraulic brake force is discontinuously increased so that a shock is generated in the vehicle and, as a result, the driver has an unnatural feeling.
Additionally, if a leak of brake fluid has occurred when the wheel cylinder pressure is increased, the wheel cylinder pressure becomes smaller than a target value. In this case, the leak of the brake force can be detected based on the change in the wheel cylinder pressure.